Volt drop calculation on spurs off of a ring.

[MrBlueSkies]

Hello,

New trainee electrician so sorry if this is a dumb question I feel confident with calculating the volt drop for a ring and radial circuit but am getting confused when in comes to incorporating a spur and and just wanted some help to confirm if I am calculating the volt drop at a fuse spur correctly or not. The ring is approximately 17.2M and there are 3 13A fuse spurs totalling 0.6M each in length (not included in length calculation for ring)

I have calculated the volt drop of my ring without the 13A spurs as followed...

Length – L​	Mv/A/m​	Volt Drop (Ring)​	Max Volt Drop​
17.2M​	18​	18 x 32 x 17.2 / 4 x 1000 = 3.99V​	5% (11.5V)​

Length – L (Spur)​	Mv/A/m​	Volt Drop (Spur)​	Max Volt Drop​
0.6​	18​	18 x 13 x 0.6 / 1000 = 0.14V​	5% (11.5V)​

Is this the correct way of calculating the volt drop of the spur by adding the volt drop of the ring to the volt drop of the cable to the spur = 4.13V?

 

[pc1966]

You have done the calculations correctly for the worst-case of both a fully loaded RFC along with a spur at maximum load.

Realistically though unless the spur is really long it can be ignored as often the RFC is below max load, the spur is not at mid-point (max VD) and well within its own max current and so voltage drop limits.

Being able to understand and perform the calculations is necessary for many cases where you have odd loads but for the vast majority of RFC applications then as long as the length falls within the limits given in the On-Site guide (Table 7.1(ii) on page 75 of the 18th AM2 OSG) you are OK.

 

[pc1966]

Just to add that generally speaking I don't like spurs off RFC as they break the excellent end-end test that all CPCs are connected, etc. However, from what you describe above it is probable a few unswitched 13A single sockets off 20A isolator switches for kitchen white goods where such an approach makes a lot of sense. In fact the Scottish building regs demand accessible isolation for such white goods.